Forrest, T.R.C.; (2010) Ordering and dynamics of strongly correlated transition metal oxides. Doctoral thesis , UCL (University College London).

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Abstract

This thesis describes a series of synchrotron based X-ray experiments on the lattice dynamics or magnetic ordering of several strongly correlated electron systems. Firstly, it will provide an introduction to the strongly correlated electron materials that were studied. After which a description of the experimental techniques used, specifically resonant X-ray scattering (RXS) and X-ray inelastic scattering (IXS), will be given. Finally the results of these experiments will be set out and evaluated. The experiments were as follows: X-ray inelastic scattering measurements on the effects of fluorine doping on the lattice dynamics of the newly discovered iron pnictide superconducting compound, SmFeAsO0.6F0.35 and its antiferromagnetic parent compound, SmFeAsO. The results from these experiments demonstrate the importance of antiferromagnetic fluctuations in understanding the lattice dynamics of this class of crystals. This result has been demonstrated for the ‘122’ class of pnictide crystals, but until now has not been shown for the ‘1111’ class of pnictides. Resonant X-ray scattering in the vicinity of the Mn L2 and L3 resonant enhancements was used to reassess the magnetic structure of multiferroic TbMnO3. The results indicate that the commonly accepted magnetic structure is modified, with additional a and c axis magnetic components. Therefore the ferroelectric polarisation in TbMnO3 arises from a phase transition between two non-collinear magnetic structures. It was previously believed that this phase transition was between a collinear and a non-collinear magnetic structure. Resonant X-ray scattering measurements were also taken on TbMnO3’s sister compound, DyMnO3. Data was recorded at the Mn K and Dy L3 resonant enhancements. Several unidentified incommensurate reflections, independent of this compound’s magnetic phases, were detected with photon energies close to the Mn K edge. What these reflections represent is still a mystery, although they do make a compelling case for further experimental work.

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